Sophie Solchenbach

Electrolyte depletion is a key driver of late-life rollover in lithium-ion batteries. However, quantitative tracking of electrolyte evolution during calendar aging remains challenging. In this study, the electrochemical aging behavior, electrolyte volume, pore volume, and pore filling ratio (PFR) evolution of automotive prototype 4695 cells (Ni-rich NMC || graphite–Si/C) are investigated during 300 d of accelerated storage at 55 °C & 100% SoC, 55 °C & 80% SoC, and 45 °C & 100% SoC. A combined approach employing periodic moment-of-inertia measurements, CT-derived jelly-roll geometry analysis, porosity measurements, and degradation-mode analysis is applied. Elevated-temperature storage results in enhanced capacity fade and resistance growth, while storage at both high temperature and high SoC additionally promotes cathode-related degradation and conducting salt depletion. In contrast, all storage conditions exhibit similar electrolyte volume losses and PFR reductions at 100% SoC. The electrolyte consumption rate, defined as electrolyte volume loss per C/20 capacity loss, is highest at 45 °C, indicating that capacity fade at 55 °C involves a larger contribution from degradation mechanisms not associated with electrolyte consumption. Consequently, electrolyte volume loss and PFR evolution cannot be directly correlated with capacity fade during calendar aging.

Fast charging of lithium-ion batteries accelerates degradation and can induce lithium plating when the negative electrode potential approaches or falls below 0   V versus Li / Li + . This study investigates the aging of multilayer pouch cells with nickel-rich positive electrodes and graphite negative electrodes under charge profiles with different minimum negative electrode potentials. Non-invasive degradation mode estimation, operando electrochemical impedance spectroscopy, and gas chromatography are combined to identify charge-profile-specific degradation mechanisms. Electrode-specific impedance growth is separated into contributions from loss of active material in the negative electrode, loss of active material in the positive electrode, and side reactions. The impedance increase associated with side reactions exhibits a strong linear correlation with loss of lithium inventory, with a Pearson correlation coefficient > 0.98 across all investigated charge profiles. This relationship is incorporated into a physically motivated equivalent circuit model using degradation-sensitive impedance factors. Parameterized with the first 200 equivalent full cycles, the model predicts electrode capacities, lithium inventory, impedance growth, full-cell voltage response, and minimum negative electrode potential. It also predicts the degradation-induced onset of lithium plating for the critical charge profile, confirmed by gas chromatography. The proposed framework enables mechanistic aging prediction and provides a basis for kneepoint prediction and aging-adaptive fast-charging strategies.

This study investigates the effect of varying electrolyte amounts on the aging behavior of lithium-ion batteries using 4695 cells containing a high-nickel NMC cathode and a graphite-silicon composite anode. Eight pore filling ratios (PFRs), ranging from approximately half-filled pores to fully saturated pores with additional free electrolyte, are examined. All cells undergo 720 cycles of accelerated aging between 0% and 80% state of charge at 50 °C with a C-rate of 0.5 C. Despite significant differences in electrolyte amount, all cells exhibit remarkably similar capacity fade and resistance increase during cyclic aging. However, subsequent fast charging tests reveal that cells with very low or high PFRs show rapid capacity loss and increased resistance due to lithium plating. For high PFRs, this is attributed to electrolyte motion induced salt inhomogeneity (EMSI), while for low PFRs, increased capacity loss results from higher initial and aging-induced resistance caused by incomplete electrode wetting. In contrast, cells with intermediate PFRs demonstrate stable fast charging performance. Furthermore, moment of inertia measurements were applied for the first time to monitor the evolution of electrolyte amount during cycle life. These findings emphasize the significant influence of aging conditions on the optimal electrolyte amount for ensuring long-term battery durability.

This study investigates multilayer and single-layer pouch cells with varying single-crystalline to polycrystalline ratios under cyclic-aging with full and partial state-of-charge windows, as well as calendar-aging. Measurements at the full-cell level include capacity retention measurements, pseudo open-circuit voltage profiling, and electrochemical impedance spectroscopy to monitor performance evolution. Post-mortem analysis of harvested electrodes employs electrochemical impedance spectroscopy and the galvanostatic intermittent titration technique. Finally, single-layer pouch cells with systematically varied single-crystalline to polycrystalline ratios are analyzed to quantify morphology-dependent aging effects. The results show that cycling between 0 and 100% state of charge accelerates capacity fade. Cycling at both high and low states of charge induces detrimental synergistic aging effects, including impedance growth and a pronounced splitting of the H2–H3 transition plateau. Restricting the state of charge window significantly mitigates degradation. Calendar-aging at 100% state of charge shows no plateau-splitting. The single-crystalline to polycrystalline ratio strongly influences aging and plateau-splitting. Single-crystalline electrodes maintain stable voltage profiles and low impedance rise, whereas polycrystalline electrodes exhibit marked overpotential increase and plateau shifts. Blended electrodes reveal heterogeneous aging behavior, which is visible as plateau-splitting in pseudo open-circuit voltage curves.

We have recently described a novel aging mechanism, namely the electrolyte motion induced salt inhomogeneity (EMSI), which was first observed in cylindrical lithium-ion cells. In the present study, large (>100 Ah) prismatic cells are cycled under different external mechanical pressures to investigate whether the EMSI mechanism also occurs in the prismatic format. It is the main finding of this study that the EMSI mechanism can indeed occur in prismatic cells, but the mechanical boundary condition plays an important role: the EMSI mechanism only occurs if the mechanical constraint prevents reversible cell swelling during charge/discharge and therefore initiates electrolyte motion inside the cell. If, in contrast, reversible cell swelling is allowed by the mechanical constraint, no (or less) electrolyte motion and therefore no (or a weaker) EMSI effect take place. Furthermore, the prismatic cells used in this study contain a reservoir of free electrolyte outside of the jelly roll. This electrolyte reservoir was found to inject LiPF₆ into the jelly roll which delays the negative impact of the EMSI effect on cell performance and aging.

This study investigates the impact of the electrolyte amount on the electrical performance at beginning of life in high energy cylindrical lithium-ion batteries. We investigate cylindrical 4695 prototype cells with varying pore filling ratios (PFRs), defined as the ratio of electrolyte volume to pore volume within anode, cathode, and separator. The PFR ranges from roughly half-filled pore volume to completely filled pore volume with additional free electrolyte. It is found that the energy and resistance remain constant as long as the PFR₁₀₀ (at 100% SoC) > 1.00. As soon as the PFR₁₀₀ < 1.00, the cell energy decreases, and the resistance increases with decreasing PFR. We also demonstrate the application of moment of inertia (MoI) measurements as a complementary technique to validate PFR calculations. Additionally, an analytical equation is developed to describe the relation between the electrolyte mass expelled from the jelly roll vs measured MoI change, isolating the contribution of electrolyte motion. This work lays the foundation for a subsequent study on the influence of PFR on the aging of cylindrical cells.

Electrolyte motion induced salt inhomogeneity (EMSI): during cycling, electrolyte moves into and out of the jelly roll, which leads to a lasting in-plane LiPF₆ concentration gradient, possibly causing secondary effects like localized lithium plating.

The electrification of the transport sector places ever-increasing demands on the energy density, fast-charging performance, and lifetime of lithium-ion cells. In this study, we investigate fast-charging of high energy density (~ 800 Wh/L) prototype cylindrical 4695 lithium-ion cells with two different (“low” and “high”) electrolyte amounts. Using pore volume calculations, computer tomography and moment of inertia measurements, we find that the volume change of the active material causes electrolyte motion into and out of the jelly roll upon cycling in the high electrolyte cells, while no electrolyte motion occurs in the low electrolyte cells. At the same time, the high electrolyte cells show a significantly worse capacity retention during fast charge cycling compared to the low electrolyte cells (24% vs. 5% capacity loss after 130 cycles). We demonstrate that a coupling of in-plane electrolyte motion with the through-plane LiPF6 concentration gradient rapidly causes a strong LiPF6 concentration gradient along the jelly roll height (= in-plane) on a cm-scale: After only 18 cycles, LiPF6 concentrations at the jelly roll edges and center (as determined by ion-chromatography) deviate by more than ± 50% in comparison to the initial average value. We term this hitherto unknown effect “electrolyte motion induced salt inhomogeneity” (EMSI). This long-scale salt concentration gradient causes a loss of cell capacity, an increase in resistance, and eventually highly localized lithium plating. Finally, we discuss the far-reaching implications of the EMSI effect on cell design and testing, not only for cylindrical cells but any large-format lithium-ion cell under high compression.

Fluoroethylene-carbonate (FEC) is a common co-solvent for high-voltage cathodes and for silicon-based anodes in lithium-ion batteries. However, FEC has a limited thermal stability when used with LiPF6 as conductive salt, and its decomposition can trigger detrimental side reactions. Here, we will examine the reaction mechanism of FEC with LiPF6, confirming that vinylene-carbonate (VC) and HF are produced at elevated temperatures. By full-cell cycling at 45°C in a T-cell setup with a micro-reference electrode (µ-RE), we can show by electrochemical impedance spectroscopy that these side reactions not only lead to an impedance increase of the anode and the cathode, but also trigger transition metal dissolution. By comparison of FEC and ethylene-carbonate (EC) as cyclic carbonate, we demonstrate that FEC has no advantage at high-voltage operation compared to EC, when employing cathode materials or cathode potentials for which no lattice oxygen is evolved. Finally, we use multi-layer pouch-cells to analyze the gassing of an EC- and FEC-based electrolyte upon extended charge/discharge cycling at 45°C, showing that the latter leads to cell bulging upon extended charge/discharge cycling at 45°C due to the oxidation of the VC formed by the thermal decomposition of FEC above ~4.4-4.5 V vs. Li+/Li